The occlusion effect is the unnatural perception of a users own voice caused by inserting a mould or a shell into the ear canal. Depending on individual geometry, the occlusion effect may cause low frequency amplification up to 30 dB. For open fits occlusion is not a problem. However, there may be situations where open fits are not feasible, e.g., due to gain or output power limitations, or when the ear canal must be sealed for protective purposes. When conventional solutions (larger vents, deep fitting, etc.) fail, Active Occlusion Cancellation (AOC) may be a viable alternative. AOC attempts to reduce the occlusion effect adding a signal in opposite phase that suppresses or cancels undesired (low) frequencies in the ear canal of the user.